Automatic telephone system



June 3, 1930. J. E. GARDNER I 1,761,115

AUTOMATIC TELEPHONE sysmm Filed Sept. 2. 1927 s Sheets-Sheet 1 2206/2 fir rfokfz zf'ardzzer' June 3, 1930.

J. E. GARDNER- AU TOMAE IC TELEPHONE SYSTEM Filed Sept. 2, 1927 s Sheets-Sheet 2' QM QM NM June 3, 1930. J. E. GARDNER 1,761,115

AUTOMATIC TELEPHONE SYSTEM Filed Sept. 2. 1927 s Sheets-Sheeta Patented June 3, 1930 v UNITED STATES PATEN'IFOFFICEY JOHN E. GAnnnnR, or CHICAGO, ILLINO S I AUTOMATIC TELEPHONE SYSTEM Application filed September 2, 1927. Serial No. 217,131.;

MyinventiOn relates in general to automatic telephone systems.

One of the objects of my invention is to provide means whereby a calling subscriber 5 can instantaneously bring about. the establishment of'a connection between his line and a desired called line.

Another object is to'provide means for pre-selecting both-the calling and called lines.

Another'obj'ect is to employ simple and efiicient switching mechanism at. the central oflice whereby a minimum-0f apparatus is used to establish -theldesired connections 1 Another object'is toprovide a highly eflicient automatic telephone exchange system in which the selections are. accomplished over one route and the talkingiconnections set up between lines over'otherroutes;

Another object is-to-render the selecting equipment usedto accomplish the "selection common toall the'subscribers lines and to release this common equipment as soon as such selection is made.

:Another object is to providecertain circuit arrangements at the central ofiice, so

that the small and simple individual.

switches associated with" the lines of both calling and calledsubscribers' areusedto establish connections between calling and called-lines.

Another object isto provide a simplified calling device at the subscribers sub-station A establishment of connectlonsv in automatic consisting of ten simple push buttonsj Another object is to provide switching mechanism at the central ofiice, which-will instantaneously respond to a" modification in an'electrical characteristic 'of the subscribers line to effect a predetermined selection;'

There are other objects of my invention which, together with the foregoing,- will be described in the detailed specification that is to follow,itaken in conjunction with the accompanying drawings. Many attempts have-been made heretofore to provide automatic selecting equipment at the central oflice which would fundtion at a high rate of speed to establish connections. However, there were certain fun- :nota'ccomplish the desired results though it: did increase the efliciency of the central damentaldifliculties he way of any solution to the problem; One of these difiiculties was the necessity for controlling the central oflice switching mechanism by means of impulses. Another, difiiculty was the ne-' c'e'ssity for employing slow acting relays.

The standard speed of impulses froma calling device in automatic telephone systems of the present dayis 12 impulsesper second. Any attempt to speed up the sending ofimpulses usually results in failure because the central office switching mechanism cannot accurately respond; to theseshorter and faster impulses. With this difliculty confronted at the outset, theproblem of speed- P ing the selecting mechanism at the central exchange cannotbe readily overcome. All the attempts that have been made hereto foreto accomplish this; result, sofar as I am aware, consisted in providing high speed 1 equipment at the central oflice of. a more or less, complicated nature. Thespeed oflthis central oflice equipment was, of course, lime ited by the fact that it had to be controlled positively in order to bring about the establishment of correct connections. However, so long as calling devices of the impulse type was employed, the-speeding up of the switching equipment in the'central ofiice did ofiice selecting equipment.

more recent attempt at speeding up the 'm'ent. As the number of lines to be connected-increases, the cost of the relay equipment advances very rapidly over that of the switch type selecting equipment and soon becomes "prohibitive. Another difiiculty encountered in the solution of this problem Wasvthe absolutenecessity for using slow acting relays or relays of different time constants. So long as slow acting relays are employed to control the establishment of the connection, certain varying intervals of time must elapse in order to permit their functioning.

Many attempts have also been made to reduce the cost of the central otfice switching equipment by employing the scheme of socalled separate setting and speaking routes. Obviously in the establishment of a connection between a calling and called subscriber, certain switching mechanisms are used which must be capable of performing a large number of selections, for example, 100 and yet when it is used in a call only one connection is made through it. It would, therefore seem desirable to establish a simple talking route after connection was set up by means of other selecting equipment. Manyattempts-havebeen made to do this but so far as I am aware, they have either resulted ,in failure or the added apparatus necessary to accomplish the result together with the additional circuit complications are more than enough to render a system of this character impractical.

My present invention utilizes the principle of instantaneous selection disclosed in my copending application, Serial No. 211,101 filed August 6, 1927, in which meansare disclosed for instantaneously bringing about the operation of the desired one of a plurality of electro-re'sponsive devices over a single circuit. Reference is here made to this prior application for a detailed explanation ofthe principles involved and manner of obtaining them.

In practicing my invention, I provided a calling device at each subscribers telephone comprising a series of ten push buttons corresponding to the 10 finger holds in the dial. These push buttons are adapted to insert thevarying amount of resistance into the line, depending upon the one operated soas to modify an electrical characteristic thereof. At the central ofiice, each subscribers line terminates in a line switch of any usual or well known type, though for the purpose of the present invention, I prefer to employ a plunger type ofline switch similar to the usual and well known Keith. In addition, these line switches are preferably of the selfaligning type so that as soon as the plunger of the line switch which is being used in a call is released, this plunger immediately comes under the control of the master switch. I also provide a master switch of the usual type for placing the plungers of all idle line switches opposite the bank contacts of the idle trunk line. The master switch is also provided with a number of extra wipers whereby any trunk line can be connected through the master switch to a common control or selecting equipment. This common control equipment consists essentially of relays, and is adapted to instantaneously respond to a variation in electrical characteristic of the calling line that happens to be connected to it.

The operation of my invention proceeds briefly as follows:

When the calling subscriber removes his receiver 114 from the switch-hook, the line switchG immediately plunges into an idle trunk. In this manner the calling subscribersline is extended to the selecting mechanism. The calling subscriber then operates twopush buttons in accordance with the two digits of the number of the desired called subscriber. By this operation the line of the called subscriber is immediately selected and his line switch is plunged in on the same trunk as the line switch of the callin subscriber. In this manner the calling ant called lines are connected together. The master switch is then permitted to operate to select the next idle trunk. Ringing current is automatically applied to the called line to summon the called subscriber The release of equipment is controlled by both subscribers.

From the-foregoing, it will be obvious that the only apparatus involved in the connection between subscribers in a hundred line system comprises the calling and called subscribers line switches and the common trunk multiple. Thus by this method of selection in a hundred line system, all the socalled connectors which have been employed in the systems heretofore are eliminated without complicating the individual mechanisms associated'with the line whatsoever. Thus by the use of the usual Keith line switchboard, anyone of a hundred subscribers. may be connected together. Since each Keith'line switch ordinarily has access to .10 trunks, ten connections between calling and calledv lines can exist simultaneously. However, since the connections are established substantially instantaneously and released in the same manner, it is probable that ten percent trunking of this character will accomplish the same results as fifteen or twenty percent trunking under the old systems.

Of course, if rotary line switches are used instead of Keith line switches, the number of trunks available are 25 and this number of trunks may serve as many as 250 lines on a 10% vtrunking basis or 500 lines, if the speed at which the equipment operates is considered.

.Referring now to the drawings comprising Figs. 1 to 3 inclusive, I have shown by means of the usual conventional diagrams suficient apparatus and circuits to enable my invention to be readily explained and understood; While I have shown battery connections to the Jarious apparatus by positive and negative ,it will be understood a common battery is employed andthe battery connections are made to it in the manner indicated.

Referring now to drawings in Fig. 1, I have shown 'a' sub-station A in the upper left hand corner of the drawing. This substation is similar to those'usually employed in automatic telephone systems and is provided with the usual transmitter 111, re-

ceiver'lla and ringer or bell 115. The calling device consists of a number of push buttons 101 and 110' inclusive. These push buttons may be of any usual or well. known type. Each push button is so arranged that when it is operated, it also brings about the operation of the spring 112 into engagement with'its working contact and the opening of the contacts 113. These push buttons bring about the insertion of a certain amount of resistance of the reslstor into the calling line circuit; The manner in which the pushbuttons 101 and 110 are con nectedto the resistor 117 is determined by,

the amount of current variation :needed for the operation of the apparatus. It is preferablethat the alteration'in resistance be such that a uniform change of conductance in line circuit is secured. Push buttons are pressed in sequence. This'will be-obvious from my copending application, Serial No.

: 205,677. The ringer or bell 150'is connected to ground through a condensor 116. If this I condenser is of sufficient capacity, no ground connectlonlsneeded. The line of the calling subscriberextends by way of conductors 120 and 121 to the central exchange where-it terminates 'in the plunger type line switch. Before reaching the line switch, a certain amount of resistance 122 to 123'is inserted in series with each conductor so as to bring the conductance of the'line of the substacontacts. The winding 12% corresponds tov what is ordinarily called-the bridge cut-off or BCOfwindi'ng, though in this instance it ,is used as a line relay. The winding 124 is adapted to bring about the operati'onof springs 129 and 130,.while plunger armature 128 remains stationary. c

y The sub-station 'A1 is similar tothe substation A just described, and the line of this sub-station terminates at the exchange in the line switch C-1 similar to the line switch C previously described.

Inlike manner, sub-stations A-2 and.

. A3 are similar to the sub-station. A and the lines associated with these sub-stations through the calling line.

terminate "at the exchange'in line switches C2and C3.

In one line switchboard, there areordinarily 100 lines, each terminating in an individual line switch similar to the line switch O." Each one of these line switches has access to lO trunks, each of the trunks being multipled together and appearing be- I tore each line switch. Each of these line switches is ofthe so-called self-aligning type; that is, as soon as the plunger arm'of the line switch is releasedafter being operated, it is immediately placed under the control; of the master switch without the necessity of the master switchmoving-to pick it'up.

.7 In Fig. 2 each one of the ten trunk lines accessible to the individual line switches extend to bank contacts inlithe master switch where they terminate. There are thus ten trunk lines, extending to the master switch bank. The trunk line comprising conductors 150 to 153, inclusive, extend to the first set of bank contacts accessible to the wipers 220 to 223,'inclusive, of-the master switch bank. Wipers 220 to 223 are adaptedtobe moved into engagement through various bank contacts by the operation of'the master switch equipment M. S; which is shown in the lower right hand cornerof Fig. 3. Re-

see 7 lays 224 to 226, inclusive, are of the usual telephone type and control the operationfof selecting relays 227 to 23 6,'inclusive. These selecting relays may be of quite sensitive construction and they are connected in at Wheatstone bridge selecting circuit formed by the resistor 237, the; resistor 238, the re sistor 239 and the control circuit extending The construction and operation of the Wheatstone bridge selecting circuit has been described indet'ail in m'ypcopendingapplication-, -SeriatNo.

211,101 Case 3. Since the pres'entinvention is shown applied'to a telephone system of lines, it is only necessary toprovide selecting equipment to select any one of the:

100 subscriberslines. "'It,'therefore, follows that only two digits will be required to bring about such selection since there are ten selections in each group; Thus, two sets of registering relays are provided to register two successive operations of any one of the selecting relays 227 to 236, inclusive. The'registering relays 241' to 250 comprise the first or tens-group of selecting relays. These relays are of the usual telephone type and are adapted .to be associated with the ion selecting relays 227 to 236, inclusive, by the operation of the switching relay 240. i The relays 251 and 252 may be of any usual or well known type and control the functioning of the selecting circuit as will appear. The

relay 253 is a dash-pot relay arranged so that it willcloseitscontacts 280 after a short interval of time. The calibration of relays.

the time required for the operation of relay 253 is determined by the average time necessary for subscribers at sub-stations to opcrate two push buttons in quick succession; that is, to completely control the selection.

The relay 253 is adapted to respond in an interval just a little longer than the average timerequir'ed for the sending of two digits. The operation of thisrelay serves to disconnect the selecting equipment to the subscribers line and render it available for the use of othersubscribers if an unduedelay is made by the subscriber in setting up his connection. The relay 239 is a inulti-contact relay of anyusualor well known type and serves to associate the selecting relays 227 a to 236, inclusive, .with the registering relays 326to 336, Fig. 3. r

In Fig. 3, the registering relays 326 to 335, inclusive, are of the usual telephone type being provided with a plurality of contacts and constituting units group of registering The master switch M. S. in the lower right hand corner of the drawing is of the usual solenoid type comprising the solenoid 364 and the controlling relays 3G3, 365 and 366. Its operation. is essentially the same as the well-known solenoid master switch controlling the operation ofa Keith line switch, serving to maintain the plungers of all idle line-switches opposite an idle trunk line. Each of theten trunk lines to which the line switches of the telephone .lines have access are provided with a relay group consisting of four relaysassociated with them. The relay trunk equipment T. E. is thus associated with the trunk line comprising conductors 150 to 153, inclusive. Thelrelay 350 is a relay for supplying talking battery to the calling and called lines. The relays 3'51 and 352 control the application of ringing current and its disconnection from the called line. The relays may be eliminated as will appear later. The relay 353 is a busy test relay and is'adapted to control the application of busy tone to the calling linein the eventthat the'called line is engaged. The subscribers at the called substation-are adapted to be summoned by the application of ringing current from the generator G. The generator G1 is provided for maintaining the alternating current cut-oil relays such as relay 351 energized until the called subscriber has responded. Ringing current from the generator Gr is adapted to be intermittently applied to called lines by the ringing interrupter RT which may be of any suitable or well known construction.

' Having previously described the apparatus shown in the drawings, I will now explain its detailed operation. In. order to do this, it will be assumed that the'subscriber at sub-station A desires to summon the subscriber at sub-station A2. In order to accomplish this result the subscriber at sub-station A will remove his receiver from the switch hook and operate his push button No. 6 twice. By the removal of the receiver at the calling station A from its switch. hook there is a circuit completed which extends from the positive terminal of the battery by way of spring 131 and its resting contact, normally closed springs controlled by spring 130, resistor 123, line conductor 121, transmitter 111 of the calllng station A, receiver 114, normally closed.

springs controlled by spring 112, switch hook springs, line conductor 120, resistor 122, spring 129 and its resting contact, hne

winding12a of the line switch C, common conductor 164, armature 261 and its back contact, conductor 313 and armature 372 and its back contact to the negative terminal of the battery. The line winding 124: is energized over this circuit and operates to attract its armature 133 thereby bringing about the operation of the springs 129 and 130. The operation of the spring 130 serves to complete a circuit which extends from ground by way of spring 131 and its resting contact, working contact of spring 130 and said spring and pull-down winding 125 of the lineswitch C to battery. The energization of the pull-down winding 125 serves to bring about the operation of the line switch plunger through the armature 128. The operation of the armature 128 also maintains the armature 133 operated. This latter operation 1S necessary because the circuit- 01" the line winding 12 1 of the line switch is opened at spring 129 and the normally closed springs controlledby spring 130. By the operation of the line switch plunger, the

subscriberresistor 123, working contact of spring 136 and said spring, trunkconductor 153, trunk conductor 170 and upper Winding of the line relay 350 to the negative terminal of the battery. The relay 350 is energized to connect ringing current over a circuit which extends from ground through the generator G through the ringing interrupter, armature 355 and its front contact, alternatback contact, armature 360 and its back contact to trunk conductor 151. This circuit also extends to the calling line. However, since there is no ground connection either on the calling line or on the trunk circuit the ing current relay 351, armature 357 and its Ian me e

result of the application ofringing current armature 128 in addition. to [operating the plunger of the line switch C also operates the spring 131 thereby opening the original circuit of the pull-down winding 125. The spring 131 is adjusted so that it is: adapted to"v be operated at theend of the stroke of the armature 128 afterline'contactssuch'as 13 1 and 136 havebeen closed so that a holding circuit is completed for the'pull down winding 1 ofthe line switch 0;

I lecting equipment.

' The operation of the plunger "of the line switch C also extends the calling line to the selecting equipment whereby there "is a circuit completed which extends from positive battery through the: secondary .winding' of the dial tone transformer, .relayl226, wiper 221 of themaster vswitch first bank contact with which said wiper is in engagement, conductor 160, spring 136 andits working contact resistor 123, calling line loop resistor 122, working contact withspring 134.

andsaid spring conductor 158, first bank contact with which wiper 220 is in engagement-and normallyclosed springs controlled by armature 228 to negative battery. The relay 226 is energized over'this circuit and operates to disconnect battery fromthepreviously tracedcircuit for itselfat the normally closed springs controlled byi armature 258 and to connectit to thesamecir cuit through the first, leg. of; the. Wheatstonebridge comprising resistor 238 :at the front contact'of this armature.- The relay 226 is thus maintained energized; The operation of the armature 258 ofthe relay 226. 7 also connects negative battery to the second leg of the bridge comprising the resistor 237. The third leg of the bridge comprises the calling'line and its extension'tothe, se-

The fourth leg of the bridge comprises theresistor 239. Other results of the operation of therrelayi 226 are that at armature 259" thevpositive' battery connection to the armature 337-0fvthe; relay 366 in the master switch isremovedand positive battery is connected torone-terminal of the fourth leg':of.thel-Wheatstone bridge at armature 259, positive'zbattery is connected to conductor281at armature 260 and its front contact andlat, armature 261 a battery connection to the' lin'e windings of all the line switches is openedw Asa result of this last: operation no :other'lineswitch suchas O can be operated'by' the removal of the receiver atv one of'the telephonesubstations momentarily; It will-be seen'that the Wheatstoiie bridge selecting circuitmight be'considered for purposes of description to; be a plurality of bridges, there being one bridge at ea'ch of thepoints on resistor 237 to which one of theiselecting relays 227 to 236,-inclusive, isjc onnected; In order that theWheatstonebridge be balanced so as to bring about the deenergiz'ation of any one of; theselecting relays 228,, for example, it

is necessary that the ratiolof'the conduc tanceof the resistor 238 to that portion of the resistor 237 extending to the point 282 be equal, to the ratio of the conductance of the calling line to the conductance-of the resistor 239, plus the conductance of that V 0 tion of the resistor 237 beyond the point 282. i The conductance of a calling line is controlled by the operation ofpush buttons and is variable within the limits of the Whetatstone: bridge selecting circuit so that by ope ratlon of the various, push buttons, anypneofthe selecting relays 227; to 236,

inclusive, may be deenergized.

. Inview of the foregoing, when the calling line loopis extendedto the selecting equipment allthe selecting relays r227 to236, in-

elusive, I are energized, Their operation serves merely to open all-the selecting circuits, 7 g

g nnother result of the operation oftheirelay 226 is that at armature 260 circuit is completed which extends from positive battery by way of'armature 260 and, itsfront contact, garmature257 and its back contact and ;relay;240- to 7 battery; The switching relay-240 isenergizedto associatethe first group of registering relays 24:1 to 251,- in- The calling bythe dial tonefrom the dialtone machine D. T.-,

that hemay operate his push buttons to establish the connection. The vsubscriber at sub-station A will thereupon depress push button-No.6. This. operation so alters the conductance of the third leg of the bridge comprisingthe calling line loop that a balance s setup whereby the relay 232 is deenergi'zed; j Under thesef conditions, the Wheatstonebridge selecting" circuit is unbalanced with respect vto eachof-the other selecting relays that is, the relays, 227 to 231, inclusive and the relays 233' to 236, inclusive, remain energized while the relay'232 .isv deenergized. 'By the deenergization of the relay- 232, thereis a circuit completed which extends from positive batteryby 'way of armature265, "armature 272 and, its back contact, working contactof spring283 and said spring and registering relay 2% to'battery;

The relay 246is energized to establish a locking circuit-for itself at armature 284 and to I iio s elusive, wlthi :the: selecting relays 227 {30,236, n ive. 1

up a

preparea selecting circuit at armature 285. Another result of the operation of the relay 232 is that at armature 275 there is another circuit completed which extends from positive battery by Way of armature 265 and its front contact, armature 275 andits back contact, and armature 256 and relay 224' to bat tery. The relay 224 is energized to complete a circuit at armature 255 which extends from positive battery by way of armature 259 and its front contact, armature 255 and its front contact, relay 225. and relay 224 to negative battery. This circuit is inefi'ective to bring about the operation of the relay 225 until the original circuit for relay 224 is opened. This is true because the relay 225 is short circuited until this occurs.-

When the subscriber at sub-station A releases his push button, the whole bridge selecting circuit is unbalanced and the relay 232 is again energized as are all the rest of the selecting relays. The operation of the armature 275 opens the original energizing circuit of the relay 224, thereby permitting the-relay 225 to be operated in series with it. The operation of the armature 271' of the relay '232 is without particular function at this time since a locking circuit has been established for the registering relay 246. The relay 225 upon operating, opens a point in the original energizing circuit of the relay 224 at armature 256, opens the circuit of-the switching relay 240 at armature 257, completes the circuit-for the switching relay 239 at the front contact of this armature. The relay 240 is deenergized to disassociate the registering relays 241 to 250, inclusive, from the selecting relays while the switching relay 239 is energized to associate the units group of' registering relays 326 to 335, inclusive, with the selecting relays 226 to 236, inclusive.

When the calling subscriber at sub-station A operates the pushbutton No. 6 in accordance with the next digit of the called subscribers number, the conductance of the calling line loop is again changed so that the Wheatstone bridge selecting circuit is balanced with respect to the relay 232 and this relay is again deenergized while all remaining selecting relays remain energized. By the second deenergization of the relay 232, there is a circuit completed which extends from positive battery by way of armature 265 and its front contact, armature 271 and its back contact, Working contact with spring 286 and said spring, conductor 308 and relay 331 to negative battery. The relay 331 is energized to attract-its armature 338 :thereby establishing a locking :circuit for itself at the spring 339 and its working contact to positive' battery on the conductor 302. The release of the push button No. 6 at the subscribers' telephone again unbalances the bridge and brings about the energization of the relay 232. The energization of the relay 232 is without particular function in this instance;-

. By the operation of the unit selecting relay 331, there isa circuit completed which extends from positive battery, relay 251, armature'285 and its front contact, conductor 319, working contact of spring 340'and said spring, conductorfl67 and pull-down winding 190 of'the' line switch C2. The line switchC+2 is individual to the line of the called sub-station A 2 having the No. 66 which the calling subscriber at substation A has just called. 'The energization of the pull-down winding 190 of the line switch C 2 brings about the attraction'of the armature 191 whereby the plunger of the line switch is caused to force the springs 192 to 194, inclusive, into engagement with their working contacts, whereby the line of the substation A-2 is extended to the trunk line comprising .conductors 151 to 153, inclusive. By the operation of the spring 193, a holding circuit is completed for the pulldown winding190 of the line switch C-2 from the release trunk conductor 152 to which positive battery is connected. This holding circuit is completed since the operation of the armature 191 brings about the operation of the line armature 195, Whereby the connections of" the line winding 196 of the line switch C2 are removed from the called subscribers line. This is the manner in which the so-called bridge cut-ofi' is secured or the line cleared of attachments.

By the operation of the springs 192 and 194 of the lineswitch C2, the called subscribers line is extended tot-he trunk conductors 151 and 153, respectively, there is a ringing circuit completed which extends from ground through the generator G ringing interrupter R-'-I, armature 355 and its front contact, alternating current relay 351, armature 357 and its back contact, armature 360 and its back contactfconductor 169, conductor 151,spring "192 and its working contact, line conductor 17?, switch hook springs of the called telephone A-2 and through the ringer and condenser to ground. The ringer at substation '-A2 is intermittently operated to signal the subscriber thereat. While this ringing circuit extends to ground if the condensor 197 is of suflicient capacity, the ground connection 'may be removed since it is well known that an alternating current circuit can be completed through a condenser..

The alternating current relay 351 in the trunk equipment is energized to complete a circuit which extends from positive battery byiway of'armature 356 and its front contact, back contact and armature 358 and upper. winding of the two step relay 352 to 'negativebattery. The upper winding of this relay has a-relativelyulow number of ampere turns and creates only a sufliciently so-called silent period. However, thisalternating current serves to maintain the-alternating current relay .351 in the trunk equip ment T. E. energized. w 5

It will be recalled that the circuit over which the pull-down winding 190 of the line switch (1-2 was operated extends through the relay 251. The relay 251 is energized in series with pull-down winding of the called subscribers line switch and operates to open a point in the circuit of the relay 252 at armature 287 and to complete a circuit at armature 288 which extends from positivebattery by way of conductor 325-, armature 37 3 and its front contact and magnet 365 to negative battery. The magnet 365 is energized to withdraw its detent 384 from the locking segment and permit themaster switch .to move-one step. This circuit is completed for the reason that the relay 366 was previously energized as soon as ground was connected to the released trunk conductor 152 over a path that extends byway of conductor 159, bank contact with which the master switch wiper 223 is in engagement, conductor 301 and relay 366 to nega: tive battery. The relay 366 is immediately energized to prepare a circuit for locking magnet 365at armature 373 and to open a point in the battery connections of the line windings of the various individual line switches at armature 372. As soon as the master switch moves onenstep its movement is stopped since the relay 366 is deenergized provided, of course, that the trunkfopposite which the plungers of all idle line-switches stand is idle. In the event that this trunk is busy, the relay 366 is again energized to advance the master switch until an idle trunk is reached. When the master switch stands opposite the tenth trunk, the springs 37 Oare itself at armature 368, establishes a locking circuit for the magnet 365 at armature 392 and at armature 367 completes a circuit for the solenoid 364. The solenoid 364 is" operated to move the m'aster'switch into-its original position so that all idle plungers ofthe individual line .switches are brought opposite the first trunk line. At this", point, the spring 369 is operated. to open the: 100k? ing :circuitofathe' relay 363' which is. deenergized to-deenergize the solenoid 364and to open the locking circuit vvof the relay 365.

The step by step movement of the solenoid master-switch thenoccurs through another cycle, w henwthe same operation again takes;

place. v e v Returning now. to the results of the;- operation of relay 251' in theconnection under discussion, the movement of the master switch one step asbefore explained positions the wipers 220 to 223 of. this switch in engagement with-the bank contacts of the secondtrunk line',"provided this trunk line is idle: This-movement of the master. switch wipers brings aboutthe opening of the circuit ofthe relay 226 in the selecting equip.-

ment and this relay is deenergized. The re- I lay 226 upon deenergizing removes battery from the Wheatstone bridge selecting circuit at-armature 258, at normally closed springs controlled by Y this armature prepares the originalcircuit for itself, removes the positive battery connection Tto; the lVheatstone bridge circuit at the armature 259, prepares the circuit for the master switch 200 at the back contact of this armature, opens the circuit-of the switching relay 239 at the armature 260, at this'same armature removes positive'battery from the locln'ng conductor 281 and at armature 261 again connects negative battery to theline windings of all the indi-I vidual line windings of all the. individual line switches suchras C. Thedeenergization i ofthe relay 226 thus releases the selecting equipment and places all line switches in such condition that they may be operated by their associated subscribers. The relay 239vis deenergized to disconnect the unit group of registering relays from removal ofrpositive battery from the conatedrwfith the next idlevt'ru nk as are the plungersof all idle line switches so that another subscriber may use the selectingequips v ment. and be connected to his desired called subscriber. over vone of .the trunkzlines accessible to the individual line switches} It will be seenithat the selecting equipment is associated with the pre-selected trunk; line during the time that the connection is setup and then released? Y Since the; subscriber at the calling station is informed by the connection of dial tone to his line when to operate his calling device and also, sincethe action of the selecting relays 227 to 236, is; instantaneously to I make 105 selecting'relays227 .to 236, inclusive. The

any'given selection, the calledline can be selected-just as fast as the calling subscriber can operate his calling device. That is, it should not take more than two or three seconds to bring about the selection of the desired called line. During this interval, the selecting equipment is rendered non-selectable to any other subscriber. In order to prevent one subscriber tying up this equipment unduly. the slow acting solenoid or dash-pot 253 is provided. This solenoid is adjusted to work within some short interval after the selecting equipment has-been associated with the calling line, for example, five seconds. Thus when the selecting equipment 'isseized by the extensionof the calling line as has been described, the relayv 226 is energized to connect positive battery to the conductor 281.

contact with which said wiper is in engagement, conductor 300, normally closed springs controlled by armature 361 in the trunk equipment T. E. and relay 353 to battery. The relay 353 is energized to open the ringing circuit at armature 360 to establish a locking circuit for itself at armature 361 to open its original'energizing circuit at the normally closed springs controlled by this armature and to apply the busy tone from the busy machine to. the calling line..

.The application of ground conductor 325 brings. about the operation of the master switch M. S. to position the plungers of all idle line switches opposite an idle trunk line in thesame manneras before. This move ment of the master switch brings'about the release of the selectingequipment in a mannerbefore described so that this selecting equipment is again rendered available for use by any othersubscriber.

The subscriber at the calling sub-station A uponhearing the busy signal will replace his receiver on the switch hook thereby releasing the equipment in a manner that will be described.

v By. this. provision, no subscriber can unduly tie up the system. The length of time that it takes the solenoid relay to operate is Very short and since the selecting equipment operates substantially instantaneously, a subscriber can complete his call to the desired calledtelephone'in a very short time. Thusthe interval that any subscriber will have towait for the selecting equipment is very-short and in all probability not appreciable. If a calling subscriber does have to wait a second or two after removing his receiver false operation isprevented as he will not operate his calling device push buttons until he hears the dial tone in his receiver signifying that he isconnected to the selecting equipment.

Returning now to the description of the establishment of'the connection between the line of the sub-station A and the line of the sub-station A-2, it will'be recalled that the ringer at sub-station A2 is being intermittently operated. W'Vhen the subscriber at this sub-station removes hisreceiver, the ringing circuit through his telephone will be opened as will the circuit of the alternating current relay 351 in the trunk equipment T. E. It will be seen that if there is a ground connection at the telephone, the removal of the. receiver from the switch hook brings about the'disconnection of the ringing circuit, the same result is brought about in the event that the ringing is being done to a condenser of sufiiciently large capacity. The relay 351 in the trunk equipment T. E. is immediately deenergized to open the original circuit of the lower winding of the relay 352. This operation removes the short circuit from the lower winding and permits it to energize in series with the upper windlng. As a result of this action, the armatures 357 and 358 of the relay are attracted. The operation of the armature 358 opens a point in the original energizing circuit of the lower winding of the relay 352. The operation of the armature 357 of this relav opens the ringing circuit so that ringing current 1s no longer applied to the line of the subscriber at the sub-station A2. The telephone substation A2 is now supplied with talking battery from the windings of the line relay 350 of the trunk equipinent T. E. The subscriber at sub-station A can now converse with the subscriber at substatlon A-2 over a circuitwhich includes only one of the trunk lines to which the line switches of each subscribers line have access The only apparatus held up for any given connection is the trunk line and the individual line switch of each subscriber. Thus ten connections can be set up between any twenty subscribers in the hundred line group, sinceeach individual line switch has access'to a common group of trunks. This provides the usual 10% trunking. In view of the speed that the connections between subscribers are established this trunking equipment will function a great deal more efficiently than the usual 10% trunking that 'is provided now in the automatic telephone exchanges. In fact, in practice, itwill give the equivalent of what 15% or 20% trunking does now. Therefore, for 7% or 10% trunkmen-1'15" ing,--a"larger numberof subscribersthan the g'ization :of the units, registering relay'33'1,

one hundred acan be=connectedin one group.

It is tobenoted that-=the trunk; line or talking'circuit over which. the connection between rsubsoribers is established, is preselected and that there 'is substantially no selecting; operation atrall #to :becontrolled during any call. The calling subscriber 'ilIl removinghis :receiver immediately :is, con-.

n at-ed to thezselectingrequipment iand -only' has' to operate two Epushwbuttonsqof rhis :a1l ing device :to bring about: the instantaneous selectionof tlieicalledzline. The called line is immediately connected to'ithe trunk "with which the calling isubsoriber is connected and the common -selectingequipment is transferred to theinext: idle trunk yasnare' all other "idleline switches "so nthat: rthe' :ne-Xt connection is pre-selected so to speak. It will be scene-that. since'ethe; called; line is selected byrelatively :CQHlPllOiLiSQCl comrnon mechanism and the iita'lking -;conne'ction between subscribers over relatively small amount of apparatus of a :simple nature that the-system is onezinvolving so -oalled separate talkingspeaking routes: This is accomplished withoutranyrofi the usual complications, maze of intricate circuits;and.-mass of apparatus; Attention: is {CllI'GOtQditO the fact that both calling 'gand-xicallezdlinesare preselected as intact the :whole connection is,

After-"the conversation between subscribers is terminated; both subscribers will ireplace their receivers I upon" their! respective switchuhooks. Wihen both? subscribers have performed this operation, theiline rre-lay 350 in trunk equipment is deenergized to remove the positive battery from the 1 release trunk conductor 152 'fOf" the trunk line comprising conductors 151 to 153, inclusive. s'Bytthe removal-of ipositi-ve battery from the "release trunk conductor "thee locking circuits :of the pull-down windings 'l25'and 190 of the line switches C and (3+2, respectively are opened and? these windings are deenergized to permit the plunger armatures 128 land l9letorestore to their original 'oondition. The-line winding armatures 133 'andil95, respectivelyyare also restoredts o sas to ;prepare circuits by which -the subscribers "on the .lines ofethe sub-stations AlandAr+-=2 may initiate their calls. Since theso-tcalled self aligning plungers are used in connection with all line switches, the line plungers :upon their release are immediatelyrplacedin align-.1 ment with the rest of the idle plungers ropposite an idle trun kfiline. 'This provision is such that "the subscribers at 'subistations A V and A'2 -'may obtain immediate service without waiting tor fmaster switchto 'be rotated to take them up as would be neces sary'if'the usualitype' plungerswere employed.

It will now be'assume'dtthat when the-line in sub-station A-2 is selected Fby the enerthat this lineis busy.v ,When the line inthe sub-station A2 is busy, theregis-n locking circuit established for i zthei .pulhdown 1 inding .190 of: the line switch G- s2. Now when the relay 339 is energized, there is, a circuit completed for relay 2551 inj series with the pull-down winding 190 otthe line switch (3-2; However, by reason of the locking positir e battery-connection, the ;=re la;y 251 is shuntedrand isrnot enerIized-i A ei-ro'uit is then effected that extends from positive battery by way of spring 34:1 and;it-s-working contact, conductor 3%, armature 287 an its back contact and relay 252 "to battery-. Relay 252-is not energized when the called line is selected if that line is;.id le forthe, reason that the relay251 is-operated to opene th s circuit {at -flIJI1iLtHEB 287. The operation :of the zrelay 252 :servesto bring about the energization of the busy test re lay- 353-intrunk equipment '1. E andrbring about the selecting operation of the master switch" M. St The operation of the master switch M. 8. brings about the release .of all the selecting equipment in the ,same manner as before. The @energiz ation of the; relay .3 53 ingthe trunking equipment 5T; informs the. subscriber; at, sub-station Ayrthat the called line is busy' in a manner that has-been explained :before; The release o-fnthe line switch; C occurs responsive, to :the fireplacement o f-"the -receiverrat the calling .Isubstation A3 in-athei-s'ame manner; as before described: 7 v V g In order to gi veia better understandings-0f the .operationrsof the system, it will new -be assumed that when: the subscriber at :sube station: A; is connected =lZO-ftll subestation AfiQ- over the trunk line comprising ,iconr, ductors 151,150 158;" inclusive ithat hthfl; "sub.- scriber sat :sub-station: 'Axfldesires to tan the subscriber 5 at sub-station {Ac-'73:, It will be assumed further that thefimaster switch is maintaining the. plunge1 s rote-all: Eidle line switches'opposite the terminals of-the trunlr linewcomprising conductors 154::to --'156,: ine elusive; "No'wpwhenthe subscriber :at subs station A.Yl removes .=his receiver, the -call-; ingt line is extended :in a unanner similar to that already described a over the; trunk;- line com-prising: conductors 154150; 156,: inclusive, 'oi zthe seleoting equipinentsof F1gs52 and 3.

In-1 -a'dditiorr, the line;- relay/- of the-strum]; equipment similar to the equipment T. E2: is energized *toi apply, ground to the release trunk thus maintaining 'itherline switchC-l operated. The -:operation-'- Ofa'UWO push buttons or the: samepushghuttontwice in calling 7 devicemf the sub-station A1 brings about two successive; balancing actions among the selecting ;relays and the. ,energi-zation of one ofthe tens relays; and zone .oiwtheunits registeri-ngj relays. :Ascircuitis thereupon closed fer pulle'downf-winding; of alineswitchC-S and this line switch is operated to extend the line of the sub-station A? into connection with the trunk circuit comprising conductors 15 1 to 156, inclusive. Rin ing current is applied to the line of the substation A3 to summon the subscriber thereat. Immediately upon the operation of the line switch C3, the selecting equipment is released and associated with the next idle trunk by the operation of the master switch M. S. This connection between the sub-station A1and sub-station A3 also includes only the trunk line and the line switches C1 and G3. The connection is released upon both subscribers replacing their receivers upon their respective switch hooks.

The manner in which connections may be made between any of the lines of the subscribers in the system will be obvious from the foregoing description.

The master switch M. S. is prevented from selecting any one of the trunk lines accessible to the various line switches when in use by-the fact that there is a ground connection on that trunk line at its terminals in the bank contacts accessible to the master switch wiper 223. At the time that the master switch is moved so that wiper 223 comes into engagement with the bank contact of the busy trunk line, the selecting equipment is idle and the relay 366 upon its energization as a result of the positive battery connection to it through the master switch wiper 223, will complete a circuit for magnet 365. to cause further movement of the master switch. This positive battery connection by which the'magnet 365 is operated extends from positive battery by way of armature 259 and its back contact and conductor 325 to armature 373.

It will be recalled that the relays 351 and 352 are employed for cutting ofi the ringing current from the called subscribers lines. However, these relays may be eliminated if it is desired. It will be seen that it is necessary to have a ground connection or a condenser connection of suflicient capacityin order that there will be a circuit for the ringing current. Consequently, if the conductor 395 is connected directly to one of the conductors of the trunk circuit that extends through the switch hook springs of the called telephone to the ringer, that, so long as the receiver remains on its switch hook at this telephone, a ringing circuit will be completed; that is, of course, after the called subscribers line has been connected to the proper trunk line by the operation of its individual line switch. W hen a subscriber at a sub-station responds by removing his receiver, the ringing circuit is opened and consequently the ring is cut off. Of course, under these conditions, there is a circuit to the ringing generator at all exchange.

times and this is undesirable. In addition, if one of the subscribers replaces his receiver on switch hook, a considerable interval before the other, a short ring would likely be applied to his line. However, if this is found undesirable, it may be prevented in any one of a number of obvious ways.

It will be seen that certain springs such as 112 and 113 are provided in each telephone so that when any push button is operated, these springs will also be operated to disconnect the transmitter and the receiver from the calling control circuit. The resistance of the transmitter and receiver is compensated for by the introduction of the proper amount of resistance in resistors such as 117. The removal or elimination of these springs will not appreciably affect the operation of the control circuit and selecting equipment.

' It will also be noted that certain resistances such as 122 and 123 are included in all lines in order to make them of a pare-determined value. This is done in order to eliminate any margin of operation whatsoever in the selecting circuit and has been explained more fully in my copending application, Serial No. 190,397. It will be obvious that the capacity of the present system may be increased by employing rotary individual line switches instead of plunger type line switches. In this event, each line switch would have access to 25 trunk lines and on a 10% basis, 250 lines could be included in the same group. By having all the line switches in one group and making all trunks available to the line switches, it is possible to provide the required trunking facilities with the use of only one switch per subscribers line. However, where the capacity of the exchange exceeds the number of trunks that have to be provided on a desired percentage basis, it is necessary to provide an incoming and an outgoing path. The efiect of this in the present system is very readily outlined by considering that another line switch board is multipled with the present one. In this instance the theory of o eration and the trunking scheme remains t e same though in this case each line switch has two paths or terminals, an incoming terminal for connecting calls and outgoing terminal for outgoing calls. The trunks accessible to each line switchboard would then be connected in multiple. However, in a single group system, it is possible to combine the incoming and outgoing terminals of a line. In order to explain how the capacity of a system of this character is amplified, we will consider a thousand line For ten percent trunking in a thousand line exchange, it is necessary that 100 trunks or connections be available for the one thousand subscribers. This is accomplished by providing one thousand line switches as the outgoing terminals of the individual line switches of the subscribers,

access to ten trunks leading to any one of the ten groups of incoming line switches. The proper line switch in the intermediate group is selected in accordance with the digits sent from the calling station, so that the proper multipled line switch is operated. In this system all the features of the present system are retained; that is, all the connecticns are pro-selected and since a push button calling device is employed at the subscribers sub-stations, the connections are completed at a maximum speed. In addition, very little equipment is used for any;

given connection. It will be obvious that the present system is adapted to party l1ne operation since it is only necessary to provide the proper ringing facilities in any obvious manner. In fact, the present ringing Y circuits shown are adapted to two partyline operation; The manner 1IlWl11Cl1 provision can be made for taking care of party line operation will be obvious to those skilled in,

the art. By employing party lines, the capacity of the group can, of course, be ampli It will be obvious that any suitable selecting circuit can be substituted for selecting relays 227 to 236, inclusive. Some of the selecting circuits which may be substituted have been shown in my copending application, Serial No. 211,101. However, the principle of selection employed and the trunking facilities provided are not limited to this particular method of control in which instantaneous selection is made or in which the selection is brought about by a modification of the electrical characteristic of the calling line, but it is equally adapted to any other system of control, such as the ordinary two wire system of control now in common use. trol can be applied to this system oiitrunkin will be obvious to those skilled in the art. With the use of an impulse system of control, the cost of the system is a great deal cheaper than present day systems of equal size and capacity through'the use of the novel trunking system herein disclosed. However, the greatest economy and the most desirable results are secured by the use of the method of control illustrated herein, whereby substantially instantaneous selection is secured without any marginal operation either of a timing nature or otherwise.

It will be understood that while I have shown and described a certain specific embodiment of my invention, that I do not desire to be limited to this embodiment but desire to secure and protect by Letters Patent all such changes, modifications and depar- The manner in which two wire contures as come within the scope 'zof' the 51331361113 claims:

' 11- 'In a telephone *system, telephone 2 lines, a trunk line common tosaidlines,means extending a calling and a called one 0f said lines tosaid trunk line,:and 32L selecting device controlling theextension ofzone :of said lines to said trunkvli-ne and means for controlling said device by varying an'electr'ical characteristic of one of: said lines through finite steps. 2.-?I;n:a telephone system, telephoneli-ne's,

switching mechanism for completingcdnnections between calling andv called lines,

push button calling devices associatedwvith of the necessary connection towards the calledline.

4; In atelephonesystem, an oifice, telephone lines terminating in said oflice, switch- 111g mechanlsm at said oifice for completing connections between calling and called lines,

push button calling devices on each line, operative to vary the resistance of that line within finite limits, and a controlling device in said office having means instantly.

operative responsive to the operation of any push button of the calling device on said line to control the switching mechanism through one stage of selection.

5. In a telephone system, an office, telephone lines terminating in said oifice, switching mechanism at said oliice for completing connections a between calling and called lines, push button calling devices on each line, and a controlling device in said ofiice having means instantly operative re sponsive'to the operation of any push button ofthe calling device on one of said lines to cause the instantaneous operation of the switching mechanism through one stage of selection. I

6. In a telephone system, a plurality of subscribers lines each terminating on a switch lndividual to it, trunk lines access-v ible to said individual switches, a selecting device comprising a group of selecting relays, means for preseleoting one of said trunk lines by aligning said individual switches opposite it and for associating said selecting device with that trunk line, means responsive to the initiation of a call on one of said lines for operating said individual switch to establish a connection over said trunk line to said selecting device to energize the relays of said device, a calling device associated with each line, means responsive to a single operation of the calling device for momentarily deenergizing one of said selecting relays to complete a stage of selection and reoperated by a second operation of the calling device to deenergize a selecting relay to complete a second stage of the section of a wanted number, means responsive to said selection for projecting a circuit to the individual switch of a particular called line to operate said switch to establish a connection with said trunk line, and means individual to said trunk line for signalling said called line.

In witness whereof, I hereunto subscribe my name this 31st day of August, 1927.

= l JOHN E. GARDNER. 

